Fix warning in config_get_global()

When config_eval() references an unknown config option, the code throws
an ERROR_CONFIG_OPT_NOT_FOUND warning.

To avoid this unwanted behavior, we now check that the config option to
replace actually exists before calling config_get()/config_get_global().
If not, the variable will be left as-is in the returned value.

Fixes #26798

core/config_api.php

@@ -633,6 +633,11 @@ function config_eval( $p_value, $p_global = false ) {
 			foreach( $t_matches as $t_match ) {
 				list(, $t_match_with_delimiters, $t_config ) = $t_match;
 
+				# Make sure the config actually exists before retrieving it
+				if( !isset( $GLOBALS['g_' . $t_config ] ) ) {
+					continue;
+				}
+
 				if( $p_global ) {
 					$t_repl = config_get_global( $t_config );
 				} else {